In optical storage, reading and writing place different requirements on the radiation spot used to scan the record carrier. Examples of optical record carriers include CD's (Compact Discs) and DVD's (Digital Versatile Discs).
When reading information from the carrier, the radiation spot size determines the mark size that is readable. It is therefore desirable that the spot size is small, so as to allow the reading of small mark sizes on carriers having a high information density.
When writing information to a carrier, the radiation spot size is less critical. Rather, it is desirable that the optical path from the radiation source to the information carrier is efficient (i.e. low loss). This reduces the power consumption of the radiation source, a factor which is particularly significant for portable applications. Further, radiation sources are typically lasers. The lifetime of a laser increases if the light path between the laser and the record carrier is efficient, as the laser can be operated at lower drive currents to generate the desired radiation intensity incident on the record carrier.
In order to obtain a small spot size for reading, it is desirable that the intensity distribution of the optical beam incident on the objective lens system is relatively flat. In other words, for reading, it is desirable that the radiation beam has a high rim-intensity (rim-intensity is the relative intensity of the beam at the edge of the entrance pupil of the objective system compared to the intensity along the optical axis).
FIG. 1 illustrates the typical optical intensity distribution across the width of a radiation beam of radius r. It will be seen that the highest optical intensity Imax is at the centre of the beam (i.e. along the optical axis in a typical scanning device), falling off relatively sharply with distance away from the beam centre. The rim intensity of the beam could be increased for reading applications by placing an absorbing spatial filter in the beam, so as to reduce transmission of central parts of the beam. However, such a filter would decrease the efficiency of the total light path, making the use of such a filter undesirable for writing applications.
It is an aim of embodiments of the present invention to provide an optical beam-shaping element suitable for addressing at least one of the problems of the prior art, whether referred to herein or otherwise.
It is an aim of embodiments of the present invention to provide an optical element suitable for providing a high rim-intensity for reading radiation beams, without unduly decreasing the efficiency of the light path for writing radiation beams.